Plant silicon (Si) content is a trait that has gained increasing attention in recent years, and is now often suggested to be a plant functional trait. This paper reviews the evidence that supports the inclusion of plant Si content within the plant functional traits framework. Plant Si contents can vary greatly among plant taxa and environments, reflecting multiplicity of roles and functions in plant biology. Plant Si uptake and accumulation also affects plant community and ecosystem properties, and thus ecological functioning and response to environmental change. Further investment in studying plant Si content as a plant functional trait and its roles in ecological processes will improve our ability to predict and mitigate the consequences of climate change.